Entities randomly appear/disappear on multiplayer server between reconnections.
Meanwhile the server constantly shows warnings similar to these:
[13:40:38] [Server thread/WARN]: Keeping entity minecraft:chicken that already exists with UUID 329fecbe-624c-47b1-8c1c-f3b0531771e5
[13:40:38] [Server thread/WARN]: Keeping entity minecraft:villager that already exists with UUID e901c66d-0a68-4a40-98d5-27459404b081
Steps to reproduce:
find a village, e.g. at (-459; 77; -925) position on seed 5000000123456
summon 70 chicken
summon 20 villager
disconnect
reconnect
After this the result is random.
only 1 or 2 villagers/chickens are present
none of the animals and/or villagers are present
all entities are present
all entities are duplicated (even my horse with diamond armor)
etc.
Look at this animated gif:
https://www.dropbox.com/s/eq06ij1ryqm0ly7/MC_19w08a_Entities_diappear.gif?dl=0
I tested it with more seeds and more animals, and results were the same.
I also tested with previous versions, and 19w07a was still NOT affected!
I tried to reproduce it in single player mode, but I couldn't.
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I can confirm entities outside of spawn chunks are generally "invisible" in multiplayer. Only restarting the Server seems to locally fix this issue, which was introduced with 19w08x. However, people noticed a similar issue is happening in singleplayer. This might be related to MC-144784.

I can also confirm this bug occurring in 19w08b, as well as 19w08a. Reloading entities seems random, and I have yet to find a consistent way to temporarily reload all missing entities. I've tried allocating more RAM to the server and client, with varying results. Changing the RAM usage of the server, then removing the main world file, generating a new world file, then replacing the old one and loading the server up again seemed to fix it for me, only if for half an hour. or however long I remained within the loaded chunks, but this could also just be coincidence. Armor stands and boats disappear and reappear, as you'd expect from entities, but even furnaces, blast furnaces, and brewing stands can stop functioning altogether. Breaking and replacing them within the chunk does not work. Named entities like pets are not immune and have the potential to disappear.

Thank you for your report!
However, this issue is a Duplicate of MC-144784.
If you have additional information, please add it to that report.
Please search before reporting, as it's likely that one exists already.
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Hello, this is not a duplicate of the other one, as the other bug report says "Seems different. Please create a new report." But it was marked as resolved and is not resolved. I moved to an area that was not my base (unloading my base) and upon return and all of my item frames, and animals at my base are gone. Pressing F3, they aren't even invisible, they're just completely gone as it shows E: 0/20 in locations where the entities were. Screenshots provided. Please re-open or advise if I should create yet another issue for this. Screenshots are attached, all chests had item frames above:
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All areas had animals in them:
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